1. Field of the Invention
The present invention relates to an electrochemical probe, and more particularly, to an electrochemical hydrogen probe having a solid-state reference electrode, which measures the amount of dissolved hydrogen in molten aluminum (Al).
2. Discussion of the Related Art
There are many ways of fabricating aluminum (Al) products, such as casting, forging and sintering aluminum powders. Among them, casting is the most popular and cheapest way of fabricating aluminum products. During the casting process, however, hydrogen is inevitably trapped in the aluminum products by a dissociation reaction of moisture in the air, where oxygen is captured by the aluminum to form aluminum oxide. Due to the large difference in the solubility of hydrogen between solid aluminum and liquid aluminum (hydrogen dissolves in liquid aluminum about 10 to 20 times more than in solid phase aluminum), dissolved hydrogen accumulates and forms voids or porosities in the final products. These defects can cause a reduction in mechanical strength and adversely affect the appearance of the final products. Therefore, the aluminum foundry industry generally employs a degassing process to remove hydrogen in molten aluminum. Since the amount of dissolved hydrogen depends on alloying elements and the level of humidity in the environment, the duration of the degassing process has to be changed from time to time in order to maintain a constant hydrogen level in molten aluminum, and hydrogen probes are an essential tool for the in-situ monitoring of hydrogen content in molten aluminum during the degassing process.
Conventional methods for measuring hydrogen content in molten aluminum may involve cutting final casting products to check the fraction of voids appearing in the cross-section of solidified aluminum ingots, which are formed by dissolved hydrogen, or involve analyzing the concentration of hydrogen in the gas which is equilibrated with argon (Ar) bubbled through the melt. The destructive nature of the former test gives rise to a sluggish throughput with a high cost of measurement. The latter needs a gas bubbling system, which also results in similar disadvantages as the former test. Alternatively, there exists an electrochemical method using a solid electrolyte for measuring hydrogen content in molten aluminum. However, this method uses a gas reference instead of a solid reference, which requires the use of a standard gas tank whenever measurements are made. Thus, this electrochemical method is inconvenient and it increases measurement costs due to the expense associated with the use of standard gas.
The conventional electrochemical sensors that use the principle of a concentration galvanic cell can measure oxygen content in molten steel or hydrogen content in molten aluminum. However, they also require a reference electrode that is in contact with a standard gas with a known gas concentration. For example, an oxygen probe in the steel industry uses air containing 0.21 atm-oxygen as a standard gas. In this way, the chemical potential of a detecting gas is set at the reference electrode to a constant value, whereby the measured electromotive force (EMF) becomes proportional to the gas content at the sensing electrode that is in contact with a gas to be detected.
Thus, most galvanic cell type gas sensors that include a hydrogen probe need a gas reference, which is inconvenient as a standard gas mixture is needed. Moreover, such gas sensors increase measurement costs due to the use of expensive standard gas.
Replacement of the gas reference with a solid phase mixture can solve the inconveniences arising from the conventional structure of the electrochemical hydrogen sensor of the galvanic cell structure. The adoption of such a solid-state reference can simplify the sensor structure, because it can be easily installed within the sensing element, but such problems as instability and gas leakage remain.
In the aluminum foundry industry, controlling the amount of hydrogen dissolved in molten aluminum is one of the most important quality control issues. As described above, conventional hydrogen measuring devices have problems such as large size, inconvenience of testing and high cost of maintenance.